In the past, edible fats and oils derived from animal sources were refined for use by physical refining methods. Vegetable oils however could not be satisfactorily refined by these methods. The many and varied impurities vegetable oils might contain would cause undesirable characteristics, such as dark colors or off-flavors or the like, in the finished oils.
At present, the most common method of refining vegetable oils is the treatment of crude oils with an alkali. Alkali refining removes free fatty acids and other acidic materials, some phosphatides, proteinaceous matter, pigments and trace metals. Until recently, most oils could not be deodorized satisfactorily unless they had been alkali refined. However, although alkali refining is suitable for preparation of oils for further processing steps, such as bleaching and deodorization, it has some serious disadvantages also.
Alkali refining typically results in a high loss of the neutral oil components of the crude oil, reducing the amount of yield of the refined oil product. In addition, the removal of free fatty acids by alkali refining results in the production of soapstock which has to be further processed in order to recover the fatty acids as a by-product. Further, alkali refining typically results in a large amount of waste water, often causing a serious water pollution problem. Alkali refined oils are additionally disadvantageous in that the finished oils require stabilizers, typically antioxidants or citric acid, or the like, to maintain acceptable flavor characteristics and prevent harmful oxidation on storage.
Alkali refining also involves a dewaxing step which increases the cost of the refined oil. This additional step removes such low temperature insoluble compounds, which are generally referred to as waxes although these compounds are not limited to compounds which are classed as waxes in a strict chemical sense. Without this processing step of chilling the oil and removing the solids formed, the finished oil would not have acceptable cold-test stability, i.e. it would not retain good clarity during storage at 0.degree. C. for 24 hours.
Various processes for the physical refining of vegetable oils have been proposed over the last few decades. Some of these have been successful with certain crude oils containing limited amounts and types of impurities. For example, U.S. Pat. No. 1,744,843 to Taylor, et al, relates to the process of bleaching animal and vegetable oils and fats and comprises adding bleaching earth and sulfuric acid. The bleaching earth is added at least as early in the process as the acid so that the fat or oil is exposed to the acid only in the presence of the bleaching earth. The mixture is agitated and the supernatant oil is drawn off and filtered through a layer of diatomaceous earth.
U.S. Pat. No. 1,964,875 to Freiberg relates to an alkali-refining process, and discloses a method of removing impurities from oils and fats in which the oils or fats are mixed with a small quantity of concentrated phosphoric acid. The mixture is then heated slowly to about 70.degree. C. and stirred, and a small quantity of cellulose is added. When the action of the phosphoric acid and the cellulose is complete, the oil is separated by filtration.
U.S. Pat. No. 1,973,790 to Appleton relates to the method of purifying non-edible vegetable oils to increase their heat stability for use in paints. The oil is purified by thoroughly mixing it with an amount of phosphoric acid which is sufficient to react with the impurities in the oil. This will cause such impurities to be precipitated and settle out of the oil in the form of a sludge. The purified oil is then segregated from the sludge.
U.S. Pat. No. 2,441,923 to Sullivan, although itself not relating to physical refining, does disclose that acid-activated adsorptive material such as Fuller's earth can be used to remove residual color from alkali refined oil.
U.S. Pat. No. 2,510,379 to Christenson relates to a method of removing lecithin, other phosphatides and coloring matter. After the treatment of the oil with an alkali, the oil is further treated with an equivalent amount of a strong acid, and resultant salt produced by the alkali and the acid is removed. Phosphoric acid and other strong acids are used to neutralize the oil (column 2 lines 35-42). The reference also shows the use of acid activated bleaching clay (column 4, lines 10-15).
U.S. Pat. No. 2,587,254 to Babayan relates to a method for reclaiming contaminated refuse palm oil used in steel fabrication. According to this method the refuse palm oil is treated with from 1% to 5% (by weight) of either phosphoric acid, sulfuric acid or hydrochloric acid in at least 50% water (by weight of oil) at some temperature from 0.degree. C. to 100.degree. C. The mixture is allowed to stratify and the oil layer is separated from the aqueous layer and bleached.
U.S. Pat. No. 2,903,434 to Gloss et al relates not to the refining of oils, but to the preparation of activated bleaching clay. Montmorillonite clay is treated with an aqueous solution of a fluorine compound and either hydrochloric, phosphoric or sulfuric acid.
U.S. Pat. No. 2,981,697 to Mickelson et al shows a process for the preparation of an acid-activated decolorizing clay. An acid-activatable subbentonite clay is treated with hydrogen chloride in aqueous solution and then washed with water until substantially free of chlorides. Then the washed clay is treated with either sulfuric acid or phosphoric acid at a temperature below about 50.degree. C. to remove the residual chlorides. Excess acid is removed from the clay, and the clay is flash-dryed at temperatures below 100.degree. C.
U.S. Pat. No. 3,284,213 to Van Akkeren discloses a process for treating triglyceride cooking oils to inhibit breakdown during heating and to prevent foaming. About 0.05 to 3.5% of concentrated phosphoric acid is added to the oil, and the oil is heated slowly to a temperature of about 100.degree. C. and stirred slowly to prevent the formation of free acid in the oil. Bleaching clay is added to the oil when the temperature has reached about 100.degree. C. and the mixture is then heated to about 120.degree. C. for about 15-30 minutes. The oil is first cooled and filtered to remove the clay and phosphoric acid material, and then heated to about 220.degree.-225.degree. C. under reduced pressure. The reference teaches that it is important to remove all of the phosphoric acid with the bleaching clay prior to the final heat treatment. A ratio of phosphoric acid to clay of 1:2 can be calculated from the examples.
U.S. Pat. No. 3,354,188 to Rock et al shows a method of refining oil in which a refining agent substantially insoluble in the oil is first combined with an emulsifying agent, and then mixed with the oil to form an emulsion. A particulate solid, such as Fuller's earth, activated clay or charcoal is then added to adsorb the refining agent and the oil is separated from the solid. Phosphoric acid is listed among the refining agents.
U.S. Pat. No. 3,590,059 to Velan discloses a process for the purification of vegetable and animal oils containing fatty acids and other impurities. The crude or degummed oil is first washed with about 1 to 5% by weight of water and the aqueous phase is separated from the resultant mixture. The washed oil is then treated with less than 0.3% by weight of organic acid, such as formic, acetic, oxalic, lactic, citric, tartaric or succinic acids or the anhydrides of any of these acids or mixtures of these acids. The moisture level of the oil is adjusted and the oil is bleached with a bleaching earth. Following this, the bleached oil is steam distilled under vacuum to remove fatty acids. The use of mineral acids, rather than organic acids, is also suggested.
Great Britain Patent No. 1,359,186 and U.S. Pat. No. 4,113,752 to Watanabe et al discloses a method of refining palm-type oils. Essentially, 0.01 to 2.0% by weight of phosphoric acid is added to the crude palm-type oil, which is substantially phosphatide-free and in which any carotinoids remain substantially in a heat sensitive condition. After mixing, activated clay is added and the temperature is increased to and maintained at about 100.degree. C. for about 5 to 30 minutes. Then the clay is separated from the oil and the oil is refined-deodorized by steam distillation at a temperature of 200.degree.-270.degree. C. with superheated steam. The calculated ratio of phosphoric acid to bleaching clay ranges from about 1:5 to 1:60.
U.S. Pat. No. 3,895,042 to Taylor relates to a process for refining crude vegetable oils as well as other fatty substances. The crude fatty substances are heated to a temperature of from about 325.degree. F. to 500.degree. F. under vacuum or inert atmosphere in the presence of phosphoric acid and acid activated clay. The resulting product is then filtered.